The invention more in particular relates to an anti-Larsen-effect arrangement in which a first means for automatic listening-level control, in addition to a variable attenuator circuit for the received speech signal, comprises a pulse-width modulator which supplies a signal comprising pulses which are pulse-width modulated by the speech signal supplied by the variable attenuator circuit, a threshold detector which, in response to the signal supplied by the modulator, supplies a compression pulse each time that it detects that the amplitude of the modulating speech signal reaches a certain threshold, said compression pulses being applied to said attenuator circuit for contolling the charge of an integrator circuit which supplies the attentuation control signal. Such first means for automatic listening-level control are described in the U.S. patent application No. 244,359, filed Mar. 16, 1981.
The invention also applies to a so-called "free-hands" telephone, provided that the sound decoupling between the microphone and the loudspeaker is sufficient to comply with the assumption mentioned in the preceding paragraph. This sound-level priority condition in favour of the user I for sound signals injected into the microphone is easier to comply with when the listening channel comprises said first means for automatic listening-level control. This ensures that regardless of the spread in levels of the signals received from the user II, which signal specifically comprises the frequency behaviour of the line, the sound level emitted by the loudspeaker is substantially constant, said level being moreover manually adjustable. Such automatic listening-level control means are known, for example from the publication: Review of the Electrical Communication Laboratories, Volume 27, numbers 5, 6, May-June 1979, pages 347-367 in the article: Model S-1P Loudspeaker Telephone Circuit Design, by K. KATO et al.
In a telephone station in general, especially in a telephone station as described in the opening paragraph, it is substantially impossible to avoid that the transmit signal is not transferred to the listening channel via the duplexer (for example, differential or hybrid transformer). This results in acoustic feedback, which may give rise to oscillations, also called the Larsen effect, in the case that the loop gain is greater than 1 at a voice frequency which corresponds to the phase condition for instability. In the situation specified in the foregoing the Larsen effect in principle does not occur when the user I or the user II (or both) are speaking, because in that case the first means for automatic listening-level control receive a speech signal and automatically attenuate said signal in such a way that the loop gain becomes smaller than 1 in most cases.
On the contrary, the problem of the Larsen effect occurs when the speech signal (signals) is (are) absent or very weak. If no measures are taken to mitigate the Larsen effect, oscillations are produced, in the last-mentioned case, even in the absence of any audible noise, because the automatic listening-level control means, which only provide an attenuation, then have a gain equal to 1, and the preset gains G.sub.e and G.sub.r of the two amplifiers of the loop (transmission and listening) are such that, despite the attenuation via the duplexer and in the air, the loop gain is greater than 1. The parasitic oscillation can therefore start and as soon as its level at the input of the first means for listening-level control exceeds a certain predetermined voltage threshold of the order of 10 mV, said means begin to influence the amplitude of the parasitic oscillation and the Larsen effect may persist, the Larsen sound-level at the output of the loudspeaker being limited to said substantially constant level selected for listening. Said limitation of the Larsen sound-level implies that the two amplifiers of the loop are not saturated and the parasitic oscillation thus produced has a substantially sinusoidal waveform. However, if the amplifiers would be saturated, which is frequently the case when the Larsen effect occurs, the waveform obtained would differ enough from a sine wave, in which case the amplifiers would no longer operate in their linear range.
When said first means for automatic listening-level control, in addition to a circuit of a known type providing a variable attenuation of the received signal, suitably comprise a pulse-width modulator and a threshold detector as described in the foregoing, and the transmission channel is connected to a second control input of the variable attenuator circuit, it is found that the Larsen effect manifests itself as a train of speech signals of limited amplitude having a frequency of the order of Hertz and such that the sound level at the output of the loudspeaker disappears substantially between two adjacent sound signals, said train giving rise to a relaxation-oscillation effect.
The object of most of known anti-Larsen arrangements is to prevent a build-up of the Larsen effect, regardless of the relative positions of the loudspeaker and of the microphone, while the transmission channel and the listening channel are never disconnected. This is obtained by respectively taking the signal from the transmission channel and from the listening channel, applying said signals to a comparator or a decision circuit, whose output controls one or a plurality of attenuators, or the amplifiers disposed in the transmission channel and or the listening channel in such a way that for all possible speech conditions to be anticipated during communication the loop gain is maintained below 1. These arrangements have the drawback that they cause considerable variations of the levels of the transmitted or received speech signal.